This application claims priority under 35 U.S.C. §119(a). on Patent Application No. 2008-138182 filed in Japan on May 27, 2008, the entire contents of which are herein incorporated by reference.
The present invention relates to a fixing apparatus used in an image forming apparatus and the like.
In an image forming apparatus and the like using an electrophotographic method that is widely applied in copying machines, laser printers, facsimiles, and the like, conventionally, a fixing method used in a fixing apparatus is generally a heat fixing method. For such a heat fixing method, a heat roller fixing method using a heat roller is generally applied.
With the heat roller fixing method, a heat roller including an internal heater as a heat source and an external surface covered with rubber or resin having good releasability, and a pressure roller are pressed against each other so that a nip portion is formed between these rollers. Then, a transfer paper on which a toner image has been formed is caused to pass through the nip portion so that the toner is heated and melted. Thereby, the toner is fused and fixed onto the transfer paper. The heat roller fixing method is suitable for increasing speeds because the entire heat roller is maintained at a predetermined temperature.
In the fixing apparatus using the aforementioned heat roller and pressure roller, the pressure roller is pressed against the heat roller so that a fixing operation is reliably performed. However, in the image forming apparatus using the aforementioned fixing apparatus, when warming up or being on standby, operations in the image forming apparatus having been stopped, or the like, if the pressure roller is maintained for a long time in a state in which the pressure roller is pressing against the heat roller, the surface of the heat roller or the like may possibly be deformed.
Thus, if the time period when the fixing apparatus does not perform a fixing operation is of a certain length, pressure applied by the pressure roller against the heat roller is cancelled so that the high quality performance of the fixing apparatus can be maintained. Accordingly, in the fixing apparatus using the heat roller and the pressure roller, a pressing means capable of switching between a pressure applied state in which the pressure roller is pressed against the heat roller and a pressure cancelled state in which such pressure is cancelled is generally used (for example, see JP 2003-280308A).
In the fixing apparatus including the aforementioned pressing means, a pressure cancelation operation and a pressure application operation are performed using the pressing means. The pressure cancelation operation is an operation for shifting from the pressure applied state to the pressure cancelled state with respect to the pressing means in the pressure applied state. On the contrary, the pressure application operation is an operation for shifting from the pressure cancelled state to the pressure applied state with respect to the pressing means in the pressure cancelled state.
In the aforementioned fixing apparatus, when the pressing means enters a state in which either the pressure cancelation operation or the pressure application operation cannot be performed, in general, the fixing apparatus determines that a failure has occurred in the pressing means and performs processing for stopping operations in the fixing apparatus in either case.
In the aforementioned fixing apparatus, when the pressure application operation cannot be performed, that is, an operation for shifting from the pressure cancelled state to the pressure applied state cannot be performed with respect to the pressing means in the pressure cancelled state, a state in which the pressure roller cannot press against the heat roller surely maintains. Accordingly, if the aforementioned fixing apparatus is caused to operate in such a state, a fixing operation cannot be reliably performed.
However, when the pressure cancelation operation cannot be performed, that is, an operation for shifting from the pressure applied state to the pressure cancelled state cannot be performed with respect to the pressing means in the pressure applied state, the pressure roller remains pressing against the heat roller.
Accordingly, if the aforementioned fixing apparatus is caused to operate in such a state, a fixing operation can reliably be performed. Consequently, even when it is determined that a failure has occurred in the aforementioned pressing means, it is still possible to perform a fixing operation depending on the state of the pressing means. Thus, there has been a demand for a fixing apparatus for such a situation.